This result is as given in the Era of 13 Sept. 1863. In Shakhmatniy Listok of Oct. 1863, it is stated that Petrov and Salter played more than 100 games, first even and eventually at odds of Pawn and Two Moves, and that Salter wrote some of them down, which might have produced the score of 5-1, which may, however, not be representative. Since Petrov was able to give Salter the stronger odds of Pawn and Two Moves, and the Era says that these 6 games were played at Pawn and Move, the result does seem reasonably plausible. According to Shakhmatniy Listok, Petrov stayed in Dieppe for 6 weeks and played with Salter daily, over a period between Sept. and Oct. 1863, but since the +4-0=2 result is given in the Era of 13 Sept. 1863, I will assume that this series finished in Sept. 1863.